Hitherto, a circular dichroism optical element formed by enclosing a fluid cholesteric liquid crystal comprising a low molecular weight material between substrates, such as glass plates, in an oriented state has been known. In the optical element, a helix axis of the liquid crystal is perpendicularly Grandjean oriented to the optical element. Of a parallel incident (incident angle 0 degree) natural light to the helix axis, about a half of a light having a certain wavelength is reflected as a right (or left) circularly polarized light, and the remaining about half of the light is transmitted as a left (or right) circularly polarized light. Its wavelength .lambda. is determined by the equation .lambda.=n .multidot.p (wherein n is an average refractive index and p is a helix pitch of a cholesteric phase). Also, the left and right of the reflected circularly polarized light is determined by the sense of the cholesteric helix and coincides with the sense of cholesteric helix.
In the circular dichroism optical element described above, because an incident light is separated to a reflected light and a transmitted light, there is a possibility of utilizing the reflected light. Thus the optical element is expected to be used in place of a polarizing film formed by adsorbing a dichroism dye, etc., to a stretched film such as a stretched polyvinyl alcohol film, etc. This is because such a polarizing film is frequently used for liquid crystal display apparatus, etc., but the light transmitting as a linearly polarized light is 50% or less of the incident light, and other light is absorbed in the polarizing film and cannot be utilized, which becomes on factor of making it difficult to increase a luminance of the liquid crystal display apparatus and lowering consumed electric power of the apparatus.
However, a conventional circular dichroism optical element is thick and heavy due to the necessity of further using substrates as described above. Thus there is a problem of hindering the attempts of reducing the weight, decreasing the thickness, etc., of a liquid crystal display apparatus. Also, there is a problem that the oriented state such as a helical pitch of a cholesteric liquid crystal is liable to be changed by the change of temperature, etc.
Cholesteric liquid crystal polymers are proposed as described in JP-A-55-21479 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 5,332,522. However, it is difficult to obtain solid materials of films, etc., having a good oriented state as low molecular weight materials, a long time such as several hours is required for the orientation treatment, and further they are poor in the practical use because the glass transition temperature is low and the durability is insufficient. Thus, in any cases, it is difficult to obtain a circular dichroism optical element of a solid state such as a film.
An object of the present invention is to provide a circular dichroism optical element comprising a solid material of a liquid crystal polymer, which is thin and light and in which the oriented state such as a pitch is hard to change at a practical temperature. Also, other object of the present invention is to provide a liquid crystal polymer being excellent in the film-forming property, capable of forming a Grandjean orientation in a good mono-domain state, capable of attaining the orientation treatment in a short time of few minutes, capable of stably fixing in a glass state, and being excellent in the durability and the storage stability.